Electronic circuitry provides complex functionality that is proving ever more useful. Electronic circuitry pervades our modern lives in areas such as communication, entertainment, travel, productivity, and the like. Advancements in electronic circuitry technology often translate to significant advances in quality of life.
One common type of circuit is the current sense circuit that measures the amount of current that flows through a switch. By measuring this current, the current may be regulated to be at an appropriate value given the circumstances, whether those circumstances warrant a relatively static current value, or a dynamic current value that is constantly varying in response to dynamic circumstances. For instance, when a low ohmic switch is connected to an external load, it is often desirable to have over-current protection to prevent circuit damage in case of overload. In addition to over-current protection, it would also often be advantageous to limit and/or measure the current flowing through the switch.
There are many conventional current sense circuits for measuring current flowing through a switch. FIG. 4 illustrates one conventional current sense circuit 400 in which a resistor 401 having resistance RSENSE is inserted in series with the switch 402 whose current is being measured. Neglecting any current leakage due to the operational amplifier 403, the current passing through the switch 402 also passes through the sense resistor 401. Thus, the voltage VSENSE at the positive input terminal of operational amplifier 403 is proportional to the current passing through the switch 402. The operational amplifier 403, transistor 405 and resistor 406 configured as shown cause the voltage at the upper terminal of the resistor 406 to be roughly equal to the voltage VSENSE at the upper terminal of the resistor 401. Thus, the current ISENSE that passes through the switch 405 and resistor 406 is approximately proportional to the current passing through the switch 402, thereby sensing the current passing through the switch 402.
One difficulty with the conventional current sense circuit 400 is that it uses the sense resistor 401. If the sense resistor 401 is large, the sense circuit 400 has high power dissipation, which increases the costs of using the circuit, potentially decreases: its lifetime, and also can contribute to heat dissipation challenges. If the resistance of the sense resistor 401 is too low, the voltage VSENSE will be too low to gain an accurate current measurement.
FIG. 5 illustrates another conventional current sense circuit 500 in which the current Id through a switch 540 (also referred to as “transistor 540”) is measured. This current sense circuit 500 permits current sensing without the use of an explicit resistor. Here, a mirror transistor 548 is used to generate a mirror current Im that is approximately proportional to the current Id through the switch 540. Factoring in the size ratio of the transistors 540 and 548, the current Id through the switch 540 may then be calculated based on the mirror current Im. In order to support this mirroring, the voltages at the gate terminals of transistors 540 and 548 should be the same, the voltages at the source terminals of transistors 540 and 548 should be the same, and the voltages at the drain terminals of transistors 540 and 548 should be the same. This is accomplished by tying the gate terminals together, and by tying the source terminals together. The drain terminals are kept at the same voltage using the operational amplifier 556 configured with feedback provided through transistor 552 as shown.
The current sense circuit 500 does not show implicit resistances that are built into the system due to metallization resistance. For instance, the transistor terminals are connected to the rest of the circuitry using a conductive material that will have some finite resistance. Similarly, the voltages provided to the current sense circuit pass through bond wires and internal conductive material as well. Such metallization resistance may be neglected in many cases. For instance, in FIG. 5, the metallization resistance may be neglected if the current is not above certain levels. However, as the currents rise, so do the IR losses due to the implicit metallization resistances. These IR losses may cause the source voltages of transistors 540 and 548 to differ even though they are shown coupled in FIG. 5 due to the presence of perhaps different implicit resistances between the source terminals and the low voltage supply. Similarly, the drain voltages may likewise be different due to different implicit metallization resistance experienced in each current path. At some current levels, the mirroring function may break down, resulting in inaccurate current sense operation.
FIG. 6 illustrates a third conventional current sense circuit 600 in which a low ohmic switch 601 (also referred to as “large transistor 601”) is composed from an array of 28 n-type field effect transistors coupled in parallel between two voltage sources labeled “Drain” and “Source”. In order to accomplish the parallel configuration, the source terminals of the 28 unit transistors are coupled together, the drain terminals of the 28 unit transistors are coupled together, and the gate terminals of the 28 unit transistors are coupled together. It is known in the art that a large transistor may best be obtained by configuring a number of smaller transistors in parallel. That way, errors in one unit transistor's characteristics may be offset by errors in other unit transistors' characteristics. Furthermore, such an array may permit for certain transistor characteristics to be obtained using a smaller layout area where such characteristics are a function of the aggregated perimeter of the transistors. An extra unit transistor 602 may be used as a current mirror transistor. The gate and source terminals of the transistor 602 are coupled in common with the respect gate and source terminals of the other transistors 601. For proper matching, the transistor 602 may be laid out and fabricated in the same transistor array as the unit transistors 601.
In FIG. 6, some, but not all, of the implicit metallization resistances are illustrated within the array 601 of unit transistors. Each of the unit transistors 601 will likely experience a different source voltage, and a different drain voltage due to the presence of the metallization resistances. Accordingly, the mirror transistor 602 may draw a current ISENSE that is not necessarily proportional to the total current passing through the array of transistors 601.
Thus, conventional current sense circuits may not provide an accurate measure of current through a switch transistor if the currents are high. That is because when the current is high enough that there are significant IR losses due to implicit resistances in the switch, the drain and source terminals of the switch transistor may have different voltages than at the respective drain and source terminals of the mirror transistor. This might be especially true if the switch transistor is composed of an array of unit transistors thereby having significant metallization resistance within the transistor itself.